1. Field of the Invention
The present invention relates to a method of forming a rotary steel output shaft for a vehicle starter motor having a planet gear speed reducing device wherein the rotary output shaft has at its one end a carrier portion for supporting planet gears and is adapted to slidably support a pinion driving device on a shaft portion of it.
2. Discussion of Background
There has been known a starter motor having a planet gear speed reducing device as shown in FIG. 2. The conventional starter motor 1 is constituted by a d.c. motor 2, a pinion driving device 6 which is slidably fitted onto a rotary steel output shaft 3 and is integrally provided with a one-way clutch device 4 and a pinion 5, a planet gear speed reducing device 7 to reduce the revolution of the armature rotary shaft 2a of the d.c. motor 2, the reduced revolution being transferred to a clutch outer member 4a of the one-way clutch device 4 through the rotary output shaft 3, an electromagnetic switch device 8 arranged at a side of the d.c. motor 2 so that the pinion driving device 6 is slidably moved on the rotary output shaft 3, and a shift lever 11 having an end engaged with a hook 9 connected to the plunger 8a of the electromagnetic switch 8 and the other end engaged with an annular member 10 attached to the one-way clutch device 4.
In the conventional starter motor 1, the planet gear speed reducing device 7 comprises a sun gear wheel 7a formed at the outer circumferential portion of the armature rotary shaft 2a of the d.c. motor 2, an internal gear wheel 7b fixed around the sun gear wheel 7a having the same center as that of the sun gear wheel 7a, and a plurality of planet gear wheels 7e which are interlocked with both the sun gear wheel 7a and the internal gear wheel 7b and which are rotatably supported by respective pins 7d fixed to a carrier portion (an arm portion) 7c through sleeve bearings 7f. The carrier portion 7c supporting the planet gear wheels 7e is formed integrally with an end of the rotary output shaft 3 as a flange perpendicular to the axis of the rotary output shaft 3. The front end portion of the armature rotary shaft 2a of the d.c. motor 2 is received in a recess 3a formed at the end portion of the armature rotary shaft 2a facing the rotary output shaft 3 so as to extend in the axial direction, through a bearing 12.
The shaft portion of the output rotary shaft 3 in the vicinity of the carrier portion 7c is supported by the boss of the internal gear wheel 7b through a bearing. The other end of the output rotary shaft 3 (i.e. the end portion at the right side in FIG. 2) is supported by a machine frame or a front bracket 13 through a bearing. A helical spline 3c is formed at the outer circumferential portion of a shaft portion 3b of the rotary output shaft 3 so as to be interlocked in its slidable range with a spline formed in the clutch outer member 4a of the one-way clutch device 4 of the pinion driving device 6 which is slidable on the shaft portion 3b of the rotary output shaft. By means of the helical spline 3c and the clutch outer member 4a, a torque of the rotary output shaft 3 reduced in its speed by the planet gear speed reducing device 7 is transferred to the clutch outer member 4a of the one-way clutch device 4, hence the torque is transferred to the clutch inner member 4c through rollers 4b, whereby the pinion 5 formed integrally with the clutch inner member 4c can be driven.
The rotary output shaft 3 in the starter motor 1 of this kind has been prepared as follows. Namely, a product having a shape similar to the rotary output shaft but having outer dimensions slightly larger than the rotary output shaft as a final product is formed by a cold forging method; the forged product is subjected to a treatment of carburization; and then the carburized product is machine-finished to obtain the rotary output shaft 3 having predetermined dimensions.
In the conventional method of manufacturing the rotary output shaft, however, much time and labor or skill are needed for finishing the carburized product having a hardened surface, which reduces processability. Namely, since finishing work for the portion having the hardened surface has to be carried out, a finishing allowance has to be very small, and grinding operation has to be carried out in its entirety of the rotary output shaft.